1. Field of the Invention
The present invention relates to a surface acoustic wave filter of a ladder type, especially to an improvement of its power-withstanding characteristics.
2. Description of the Related Art
Recently, the market of mobile communication apparatus represented by, for example, portable telephones are rapidly expanding, and the demand for small and efficient parts to be used in these apparatus has become strong. Because the high power-withstanding characteristic is required in an antenna switch located in a front end of radio frequency circuit of the mobile communication terminal, there has been conventionally employed a dielectric filter. However, in order to make the apparatus further smaller it is necessary to replace the dielectric filter with a surface acoustic wave (referred to hereinafter as a SAW) filter. On the other hand, width of electrodes of the SAW filter becomes minute as the operation frequency rises; accordingly, further power-withstanding characteristic has come to be required.
The SAW filter of a ladder type is a filter in which SAW resonators having mutually different resonance characteristics (single-pair terminal resonator) are arranged in the parallel arm and in the series arm, where insertion loss can be decreased very much owing to the use of this ladder type SAW filter compared with the case using a SAW filter of a transversal type in which the comb teeth electrode pair is connected in multiple stages. Details of the SAW resonator will be explained later.
Impedance of the resonator is zero at the resonance frequency fr, and is maximum at the antiresonance frequency fa. On the contrary, the admittance is maximum at the resonance frequency fr, and is zero at the antiresonance frequency fa. That is, the resonator is tuned in two ways.
Such resonators are connected in an L shape arrangement so as to constitute a two-pair terminal resonator, where the resonance frequency of resonator Rp in the parallel arm and resonator Rs in the series arm are set such that the antiresonance frequency faP of resonator Rp of the parallel arm conforms substantially to the resonance frequency frs of resonator Rs in the series arm, Whereby, there is formed a band-pass filter having a center frequency which is the resonance frequency frs of resonator Rs in the series arm. Then, a band-pass filter characteristic which satisfies the specification, such as of portable telephones, is accomplished by connecting the two-pair terminal resonators in multiple stages to form a ladder structure as shown in an equivalent circuit of FIG. 3A.
When the SAW filter is built in a radio frequency circuit, such as of portable telephones, an electric power withstanding characteristic is required corresponding to the maximum transmitting power of the apparatus. A consideration to Joule heat is severely required, particularly in the use in an antenna duplexer (a transmitter/receiver switch) to which the transmitting power output from the output amplifier stage is applied. The antenna duplexer is a device for sharing the antenna by transmitter and receiver by the utilization of the difference between the transmitting frequency and the receiving frequency, and consists of a transmitter band-pass filter in which the transmitting frequency band is in the pass-band and a receiver band-pass filter in which the receiving frequency band is in the pass-band.
In the prior art SAW filter, there was a problem in that the temperature rose in the filter chip which formed the resonator and the characteristic was apt to deteriorate when a signal was input at an attenuation band (a stop band) which is at the lower frequency side in the filter characteristic (bandpass characteristic). That is, power-withstanding characteristics at the attenuation band at the lower frequency side was low compared with power withstanding characteristics at other frequency bands.
Therefore, when an antenna duplexer employed in, for instance, AMPS (Automatic Message Processing System), which is an analog portable telephone system, adopted in North America and South America, was composed of a prior art SAW filter, the receiver filter, to which the signal of the frequency band (transmitting signal) having low power-withstanding characteristics was input, was deteriorated earlier than the transmitting filter, because the transmitting frequency (824-849 MHz) is lower than the receiving frequency (869-894 MHz).
On the other hand, the current route varies depending on the input signal frequency as shown in FIG. 2, which schematically illustrates the relation between the filter characteristic and the current routes in the SAW filter. That is, in principle, the signal current of the frequency of pass band A flows in the series arm, and the signal current of the frequency of attenuation band B1 at the lower frequency side flows chiefly in the parallel arm. The signal at the high frequency side attenuation band B2 hardly flows into any resonator in the filter.
Moreover, when the currents in each stage are compared in the ladder type filter, the currents become smaller in the order going from the first stage at the input side to the latter stage. That is, the relations are expressed by formulas (1) and (2), where the currents flowing in each resonator RS1, RS2, Rp1, Rp2 and Rp3 are Is1, Is2, Ip1, Ip2, and Ip3, respectively, as shown in FIG. 3A. EQU Is1&gt;Is2 (1) EQU Ip1&gt;Ip2&gt;Ip3 (2)
Therefore, it is considered that the decrease in the Joule heat caused from the current Ip1 flowing in resonator Rp1 of the parallel arm at the input side first stage is especially effective in improving the power withstanding characteristics of attenuation band B1 at the lower frequency side.
Here, impedance Z of resonator Rp1 is represented by formula (4) according to FIG. 3(B). EQU Z=Z1*Z2/(Z1+Z2) (4)
where Z1=-j/.omega.C.sub.0 and
Z2=R+j.omega.L-j/.omega.C.sub.1
And, resistance element Zr (real part of impedance Z) involved in the Joule heat is shown by the formula (5). EQU Zr=R/(.omega.C.sub.0).sup.2 [R.sup.2 +(.omega.L-1/.omega.C.sub.0 -1/.omega.C.sub.1).sup.2 ] (5)
In resistance element Zr, resistance R can be decreased by appropriately selecting opening width x of comb teeth electrodes 111 shown in FIG. 1 and the number of the pairs (Japanese Provisional Patent Publication HEI6-29779).
However, it was confirmed by the present inventors that the resistance of the wiring conductor formed with a thin film largely took part in the heating of the filter chip according to the below-described experiments.
In the parallel arm of the input side first stage, actual resistance element ZR1 which contributes to the heating of the filter chip is represented by formula (6). R1 in formula (6) is resistance of the part between the resonator and the bonding wire in the above-mentioned wiring conductor. EQU ZR1=Zr+R1 (6)